Jacob and Wilhelm Grimm are the authors of Grimms' Fairy Tales, a collection of mostly familiar stories full of royalty, evil witches, and animated nature. In one sense, you should not have to prove that a fairy tale is not true because the very definition of a fairy tale proves it: "a false story that is meant to trick people." Clearly a fairy tale is not true and the characters in them are not real and do not exist.
Nevertheless, if you have to prove that these stories are not real or true, here are a few suggestions you might use in your speech to do that.
- The settings of these stories are not real. Nearly every one of these fairy tales is set in an unnamed place, such as a forest, a garden, or a castle. These places have no specific names because they are not real places.
- The objects in these stories are not real. Consider the bird made of gold in "The Golden Bird" who could not fly if he were truly made of gold, the turnip as big as a cart (before the invention of Miracle-Gro) in "The Turnip," and a spinning wheel that turns straw into gold in "Rumpelstiltskin."
- The actions of the characters are not real. For example, the fish that can talk in "The Fisherman and His Wife," the king who eats a bite of white snake and can suddenly understand the languages spoken by animals in "The White Snake," and the talking, knocking-on-the-door frog who turns into a prince in "The Frog-Prince."
You get the idea for this approach--simply point out that none of the primary elements of these stories can be real and show that, for example, a bird made of gold would not only be too heavy to fly but too weak to flap his heavy wings (let alone all the other parts of the bird which, if they were gold, would eliminate the possibility that a bird could even be alive). This requires taking a few examples in each category and showing that they are not and cannot be real.
Another idea is to discuss fairy tales in their historical context and purpose, distinguishing them from legends and folk stories which were generally rooted in some kind of truth and fables which are meant to teach some kind of a lesson. In contrast, fairy tales were designed to stir the imagination and were set in times long ago. Consider the etymology of the word.
[W]ith the common beginning "once upon a time" ...a fairy tale or a märchen was originally a little story from long time ago, when the world was still magic. (Indeed one less regular German opening is "In the old times when wishing was still effective".)
Clearly the meaning of "fairy tale" is intricately connected from the beginning with wishing and magic, not reality and actual truth.
You might even think about taking one specific fairy tale and talking about what life would be like if everything that happened in the story were real and true. What things might be talking to us regularly, for example, or what parts of our world would be different (back to giant vegetables and birds made of gold). It would not take long for these things to seem pretty ridiculous as realities.
Whatever approach you take, it seems to me that your first argument has to be that just because fairy tales do sometimes contain things, people, and places that could be real, they are not truly real. In other words, the argument that says "Snow White" contains poison and poison is real therefore the story is real, is not valid. Think of all the other fictional stories which use real places and real things but are not real. It is an easy case to make.